Each year, the emission of air toxics from combustion sources is being subjected to increasingly strict regulations. These regulations include not only existing regulations, such as the U.S. Clean Air Act as amended in 1970, 1977, and 1990, the National Energy Act, and the National Primary and Secondary Ambient Air Quality Standards, but also pending regulations that will require the removal of certain air toxics from utility plant flue gas. Title III of the Amendments governs air toxics. As used herein, an “air toxic” refers to the 189 chemicals listed in the Clean Air Act. Air toxics are present in the flue gas of combustion sources and appear both as particulate metals such as nickel, arsenic, and chromium in flyash particles and as vapor phase metals such as mercury, selenium, halogens, and halides and organic vapors. Vapor phase air toxics are commonly present in flue gas in trace concentrations of parts per million or less and therefore can be difficult to remove to comply with pertinent regulations.
Several systems have been developed to remove trace quantities of air toxics from flue gas. The systems have had varying degrees of success.
In one system, activated carbons and carbons treated chemically to produce sulfide or iodide compounds with mercury are injected into a gas stream ahead of a particulate collection device, such as a Fabric Filter (FF), ElectroStatic Precipitator (ESP), Spray Dry Absorber (SDA), and Flue Gas Desulfurization (FGD) device. The activated carbon is typically introduced into the gas stream by blowing the carbon, in a dry particulate form, into the gas stream. While in flight, the carbon reacts with the entrained air toxics and binds the air toxics to the surfaces of the carbon. The air toxic-containing carbon is then removed by the FF, ESP, SDA, and/or FGD.
The carbons used for the mercury removal process are generally of high surface area and ground by the manufacturer to a small particle size, typically in the range of 10 microns to 100 millimeters. Manufacturers make different grades of carbon depending on the specific properties desired for a particular application. Activated carbon is manufactured in a finished form at centralized manufacturing locations. The material is then shipped in bulk form over long distances to end user locations using various means, such as supersacks, rail car and truck.